Various kinds of carbon fibers which are obtainable by vapor-phase growth (or vapor deposition) processes are inclusively referred to as vapor-grown carbon fibers. Vapor-grown carbon fibers has some advantages such that those having a high aspect ratio are easily obtainable, and therefore VGCF has actively and energetically been studied, and a large number of reports on these production processes have heretofore been published. Carbon nanotubes (i.e., a kind of carbon fibers having a fiber diameter on the order of nanometer(s)) which have particularly attracted much attention in recent years, may also be synthesized by appropriately applying the vapor phase-growth process to the production of the carbon nanotubes.
FIG. 1 is a schematic sectional view showing an example of the reactor for continuously producing carbon fibers by utilizing a vapor phase-growth process. In an example of the general production procedure using this apparatus, CO or hydrocarbon such as methane, acetylene, ethylene, benzene, and toluene is used as the raw material. When the raw material such as hydrocarbon assumes a gaseous state at normal (or ordinary) temperature, it is supplied to the apparatus in the gaseous state together with a carrier gas. When the raw material such as hydrocarbon assumes a liquid state at normal temperature, the raw material is vaporized and supplied to the apparatus in a mixture thereof with a carrier gas, or the raw material is sprayed in the liquid state toward the heating zone in the apparatus. As the carrier gas, it is possible to use nitrogen gas as an inert gas or hydrogen gas having a reducing property. As the catalyst, it is possible to use a supported catalyst comprising a carrier such as alumina, and a metal supported on the carrier, or an organometallic compound such as ferrocene. In a case where the supported catalyst is used, the supported catalyst is preliminarily placed in the reaction zone in the apparatus and is heated so as to effect a predetermined pre-treatment, and then the raw material such as hydrocarbon is supplied to the apparatus so as to cause a reaction (in the example shown in FIG. 1); or the pre-treated supported catalyst is supplied from the outside of the reaction system to the apparatus continuously or in a pulse-wise manner, to thereby cause a reaction. Alternatively, it is also possible to feed to the heating zone of the apparatus an organometallic compound such as ferrocene, as a homogeneous-type catalyst precursor compound, together with a raw material such as hydrocarbon continuously or in a pulse-wise manner, so that carbon fibers are formed by using as the catalyst the metal particles which have been produced due to the pyrolysis of the catalyst precursor compound. The resultant product is collected to the inside of the heating zone or the collector 4 (in FIG. 1) disposed at the terminal of the heating zone, and is recovered after the completion of the reaction for a predetermined time.
The processes for producing carbon fibers by utilizing a vapor-phase technique may be roughly classified into the following three kinds of processes, in view of the method of feeding a catalyst or a precursor compound for providing the catalyst.
(a) A method wherein a substrate or boat comprising alumina or graphite which carries thereon a catalyst or precursor compound therefor is placed in a heating zone, and a hydrocarbon gas to be supplied from a gas phase is caused to contact the substrate or boat;
(b) A method wherein particles of a catalyst or precursor compound therefor are dispersed in a liquid hydrocarbon, etc., and the particles are supplied from the outside of the reaction system into a heating zone continuously or in a pulse-wise manner, so that the particles are caused to contact the hydrocarbon at an elevated temperature; and
(c) A method wherein metallocene or a carbonyl compound which is soluble in a liquid hydrocarbon is used as a catalyst precursor compound, and the hydrocarbon containing the precursor compound dissolved therein is supplied to a heating zone so that the catalyst is caused to contact the hydrocarbon at an elevated temperature.
Among these, an intended product can stably be obtained continuously, particularly by using the above method (c), and therefore, VGCF can be produced in an industrial scale by using this method (c). Further, with respect to the above method (b) which enables the continuous production of the carbon fibers, there have been reported a method wherein a suspension to which a surfactant has been added is used, for the purpose of stabilizing the ratio of the quantities of hydrocarbon and catalyst to be fed to the reaction system (JP-B (Examined Patent Publication) 6-65765; Patent Document 1); and a method wherein fine particles of a catalyst having uniform particle size of nano-order which have been synthesized by utilizing a microemulsion, are suspended in a hydrocarbon such as toluene, and the resultant suspension is continuously supplied to a heating zone, to thereby synthesize single walled carbon nanotubes (Kagaku Kogyo Nippo (The Chemical Daily) dated Oct. 15, 2001; Non-Patent Document 1).
[Patent Document 1]                JP-B 6-65765        
[Non-Patent Document 1]                Kagaku Kogyo Nippo dated Oct. 15, 2001        
However, the above method (a) includes steps wherein a catalyst or precursor compound therefor is applied to a substrate, the catalyst or precursor compound is, as desired, subjected to a pretreatment such as reduction thereof, then carbon fibers are produced by using the catalyst or precursor compound, and the resultant carbon fibers are taken out from the reaction system after the temperature is decreased. Further, these steps should be conduct independently. Accordingly, it is difficult to continuously obtain the intended product, and therefore the productivity thereof is poor. In addition, it is necessary to adopt a large number of steps such as preparation of the catalyst, application thereof to the substrate, the reduction of the catalyst as the pre-treatment into a metal state, the production of carbon fibers, and the recovery of carbon fibers from the substrate. Accordingly, this process is not economically advantageous.
On the other hand, in the above method (b) or (c), carbon fibers can be produced continuously, but these methods have a tendency that a sufficient amount of carbon fibers cannot be obtained unless the catalyst or precursor compound therefor are used in a large excess amount, as compared with the amount thereof which is required. Accordingly, not only an expensive catalyst or precursor compound therefor tends to be wasted in these methods, but also it is necessary to adopt a step of removing the by-products which are originated from the large-excess addition of the catalyst, etc., and such an additional step considerably impairs the economical advantage of these methods. As described above, a process which is capable of producing a large amount of vapor-grown carbon fibers inexpensively, has not been developed yet, and this inhibits the industrial-scale production of vapor-grown carbon fibers.